This invention pertains to removal of radar absorbing coatings, and other coatings that absorb microwave energy, from a substrate using microwave energy.
The processes that have been used to remove radar absorbing material coatings from substrates, particularly from aircraft and ships, include mechanical abrasion; grit blasting, including using dry ice as the abrasive; mechanical scraping; heat lamps; and continuous and pulsed lasers. Conventional mechanical abrasion using conventional abrasives and wire brushes are rather ineffective in removing some of the coatings which are based on rubbery polymers, such as urethanes used for all-around purposes, neoprenes used for their weather resistance, nitrile rubbers used for their fuel and oil resistance, and fluoro-elastomers used for their excellent operating temperature range. In addition, this method produces substantial volume of waste products, including mixtures of abrasives and coating residue, requiring special disposal. Grit blasting, which has been done with conventional abrasives, plastic abrasives and, most recently with crushed dry ice as the abrasive, is not very effective against the rubbery coatings, suffers from problems with waste and with the use of dry ice which results in evolution of substantial amounts of carbon dioxide emissions as well. Mechanical scraping can be done successfully but it is very labor-intensive, requires skilled workers, and carries with it risks of significant damage to the substrate to which the coating is or was bonded.
The rest of the methods are either thermal or thermomechanical in nature. Heat lamps are limited in maximum power density deposited in the coatings and heat the coating from the outward surface inward, depending on heat conduction through the low conductivity coating to degrade the bulk of the coating. As a result, significant time is involved in raising the entire coating thickness to a temperature that would degrade it, and the substrate is consequently heated as well. Such techniques also result in large emissions of volatile organic compounds as the outer layer of the coating is heated to very high temperature. The use of continuous lasers, such as the carbon dioxide lasers, has the advantage of permitting higher power density than with heat lamps but has most of the same disadvantages as well. Pulsed lasers, especially very short pulse systems, can be used to remove material by ablation where the local power density is so high that the material within the beam is vaporized or spalled off by vapor generation. However, this is still a surface process and the laser radiation penetrates only a very short distance, of approximately 1 micron, into the coating resulting in a relatively slow material removal process, that has to work its way down through the coating thickness and is limited by the ablation products blocking the incident radiation, rather then removing it completely in one operation.
As the name implies, radar absorbing materials or microwave absorbers are coatings whose electrical and magnetic properties have been altered to allow absorption of microwave energy at discrete or broadband frequencies. These materials are typically produced by using existing materials and altering their dielectric and magnetic properties. For purposes of analyses, the dielectric properties of a material are categorized as its permittivity and the magnetic properties as its permeability. Both terms are characterized as complex numbers with real and imaginary parts. Common dielectric materials used for absorbers, such as foams, honeycombs, netting, plastics and elastomers, have no magnetic properties, giving them permeabilities of 1. Magnetic materials, such as ferrites, iron and cobalt-nickel alloys, are used to alter the permeabilities of the materials. High conductivity materials, such as carbon, graphite and metal flakes, are used to modify the dielectric properties.
The increased need for radar absorbing materials has resulted from two ongoing developments: the first being the greater number of electronics systems being incorporated into vehicles, including aircraft and ships, which has resulted in a corresponding growth in electromagnetic interference. These problems include false images, increased clutter on radars and reduced performance because of system-to-system coupling. The second development being that there are ever greater requirements for reduced radar cross section of weapon systems. Not only is there a need to reduce a vehicle""s signature, but equally as important is the need to reduce the signature of various electronic warfare systems and payloads attached to these vehicles. Radar absorbing materials play a key role in stealth technology and their use is a major factor in radar cross section reduction.
It is an object of this invention to remove radar absorbing materials and other materials from substrates, such as aircraft and ships, by the use of microwave energy.
It is another object of this invention to minimize production of volatile compounds and other hazardous waste and byproducts resulting from removal of coatings from metallic and non-metallic substrates.
Another object of this invention is to minimize or eliminate thermal and mechanical damage to substrates to which coatings are secured.
Another object of this invention is to remove a coating from a substrate quickly by the use of microwave energy.
These and other objects of this invention can be achieved by directing microwave energy at a coating disposed on a substrate to degrade the coating and then removing the coating from the substrate.